Diode detection system



Dec. 10, 1940. c, TRAvis DIoDE DETECTION SYSTEM Filed July 9, 1932 2 Sheets-Sheet l Dec. 10, 1940.

c. TRAVIS 2,224,707

DIODE D'ECTION SYSTEM Filed July 9, 1932 2 Sheets-Sheet 2- INVENTOR CHARLES TRAVIS l ATTORNEY.

Patented Dec. 10, 1940 UNITED STATES DIODE DETECTION SYSTEM Charles Travis, Philadelphia, Pa., assigner to Radio Corporation of America., a corporation of Delaware Application July 9, 1932, Serial No. 621,598v

11 Claims.

My present invention relates to rectification systems, and more particularly to an improvement in a detection system for a radio receiver of the type employing a diode as a rectifying de- Vice.

One of the main objects of the invention is to provide a diode detection system wherein there is avoided the distortion arising at high modulation values due to the difference between the alternating current and direct current loads on the diode.

Another prime object oi the present invention is to provide a radio receiver employing a diode rectier for automatically controlling the gain of amplifier stages preceding the rectifier, the anode oi the diode rectier being biased in a positive direction with respect to the cathode of the diode in order to avoid distortion arising at high modulation.

Another important object of the present invention is to provide an improved type of automatic gain control for a radio receiver, which control includes a diode detector between the high frequency and low frequency ampliiiers, the anode of the diode detector being maintained biased in a positive direction in a predetermined manner with respect to the diode cathode throughout operation of the receiver.

Still other objects of the invention are to improve generally the eiiiciency of diode detector arrangements, and to particularly provide an arrangement of this type which is notl only durable and reliable in operation in a radio receiver, but economically manufactured and assembled in a radio receiver embodying automatic gain control.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into eect.

In the drawings,

Fig. 1 shows a graphic analysis of the problem sought to be solved by the present invention, and the solution of the said problem,

Fig. 2 graphically shows a signal wave before using and after using, the present invention,

Fig. 3 diagrammatically shows a radio receiver embodying the present invention.

Referring now to Fig. 1 which graphically analyzes the operation oi the present invention, as well as the need for it, the circuit shown at (a) in Fig. 1 represents an arrangement for securing curves 0 to 10 inclusive. The curves are plotted with abscissae E in volts, and I in micro- .5 amperes as ordinates. The circuit shown at (a) in Fig. 1 represents an arrangement for securing the 11E characteristics of the diode I, this diode being for the purpose of illustration a 27'` type tube having its grid and anode strapped together, 10 for various inputs of radio frequency voltage impressed upon the resonant input of the diode I.

The source oi input voltage for the diode I comprised a screen grid tube 2 having an alternating current source 3 coupled between the grid 15 and cathode thereof, the values of E being measured between the cathode and upper terminal of the resistor' R1. The values of I were measured between the grounded side of the cathode of diode I and the lower terminal of the 20 resistor R1.

In obtaining the curves 0 to 10 inclusive in Fig.

1 the resonant circuit in the input diode I in the circuit diagram a was adjusted to give 50,000 ohms resonant impedance, and the radio `fre 25 quency inputs were measured at the grid of the preceding radio frequency amplifier tube, on the basis of the voltages they would give over the tuned circuit alone. Thus, curve 10 represents an input of 100 volts peak across the tuned circuit 30 alone, but if the diode I is connected on with a leak R1=100,00O ohms (represented by the line OBB), the conductance oi the diode I reduces the tuned impedance to about half itsvalue, so that only about 50 volts peak appears acrossthe '35 diode. This condition is represented by B', where the direct current voltage across the 100,000 ohm leak is about volts. This is an advantageous way to show the efficiency and operation characteristics of the diode.

The other curves represent tenth steps of this input down to zero all taken at the grid of the preceding tube 2. The output of the diode I is assumed to be the leak R1, eiectivefor both direct current and alternating current and 45 coupled to this by a condenser, the leak R2 being effective for alternating current only.` Rzrepresents the 4combined value o-f a grid leak fora following audio stage, filter resistors for automatic volume control, and the like. In` what follows the effective resistance to direct current will be called Rdc, and that for alternating cur- I'elll'r Rao. Y I i In Fig. 1 the resistors R1 and R2 have'each been assumed equal to 200,000 ohms so" that the direct current load on the diode Rdc is 200,000 ohms while the alternating current load Ric is 100,000 ohms. For the ordinary operation (without bias) it is assumed that an unmodulated carrier is impressed, represented by the curve 5. The

generated direct current and alternating current voltages are then given by the coordinates of the point A, where the line OA has a slope equal to the reciprocal of 200,000.

If the carrier is now modulated the operating point A will move up and down along the line A1, A, A2 following the audio frequency cycle; for the line A1, A, A2 has a slope equal to-the reciprocal of 100,000 ohms, the alternating cur-V rent load on the diode. The important thing to note is that the locus of A is the line A1, A, A2 and not the direct current load line OA1.

At about 70% modulation the point A swings from A1 to A2, and there is little distortion of the shape of the modulating wave. If, however, the modulation is increased beyond this value, it is seen that the operating point cannot swing below A1, as this is the current cut-oi point. The negative peaks of the modulating wave will be chopped off at this point. This is shown to scale in Fig. 2 for the case of 100% modulation.

Thus, in Fig. 2, the solid line X shows the effect on a 100% modulated signal wave of the cut-01T action at point A1 in Fig. 1. in other words, when the diode is employed for detection as shown at (a) of Fig. 1, distortion, of the type wherein the negative peaks of the modulating wave are chopped 01T, results. 1t is recognized that the actual distortion will not be Nas bad as that shown in Fig. 2 for the reason that the direct current component of the unsyminetrical wave will tend to move the point A and the line A1, A, A2 with it, upwards and to the left, thus somewhat reducing the cut-01T portion. However, quantitatively the eilect shown by the solid line X of Fig. 2 remains, .and results in severe distortion 'audible 'in the loud speaker as a buzzing or rattling sound. It has been proposed to ameliorate this condition by. making R1 small compared to R2, Vso that Rdc and Ras are more nearly equal than in the case illustrated. This, of course, means thatthe eiliciency of the diode is seriouslyiinpaired.

According to the present invention, however, a direct current bias is inserted in series with the resistance R1 in such a sense` that the anode of Ythe diode I is made more positive than it otherwise would be. That is the anode is biased in a less negative direction with respect to the cathode. This is shown in the circuit diagram designated by the letter (b) i-n Fig. 1. It can Abe shown that the bias in the positive direction does not change the curves 0 to 10 of Fig. 1 in any way, but is simply equivalent to moving the origin of voltage coordinates from the point 0 to new point 0', where 0,0' represents the biasing voltage in the positive sense. This moves the operating point to B for the unmodulated carrier, and for modulation B may swing over the whole range from 0 to 10 without overpassing cut-01T.

If the carrier is increased beyond 5 the eiiect will again appear. In other words, the bias should be adjusted to the carrier strength. Practically, however, this is unimportant. Thus, in Fig. 1 if the carrier is doubled (to curve 10) without increasing the 22% volt direct current bias used, the load line will be B" A1, which still covers 85% modulation without cut-oil. In other words, a iiXed bias will more or less compensate for the diierence between Rae andRds over a range of carrier strengths, and the results will be better than if no bias were used, even if the bias is not suiiiciently great for exact compensation.

The dotted .line Y of Fig. 2 shows the chopped peaks restored by operating at B of Fig. 1 which is done by biasing back -|-221/2 volts. An advantage of this type of biased diode detector in automatic volume control Work involves the fact that with the bias the change in direct current developed across R1 is not proportional to the carrier strength. As the latter is increased from zero the direct current rises slowly at first, and then more rapidly. This is an ideal condition -for automatic volume control operation, since it automatically provides a certain amount of delay without the use of another rectifier element.

In Fig. 3 there is shown a circuit diagram of a conventional type of radio receiver utilizing the present invention. The receiver comprises a pair of tuned radio frequency amplifier stages, the rst stage including the screen grid tube I0, and the second stage including the screen grid tube I I, the 'detector in this case comprising the diode tube I of the type shown in Fig. 1. The input circuit of each of tubes I0, II and I includes a variable tuning condenser I2, and the grounded rotors of each of these condensers are arranged for mechanical uni-control, in any desired fashion, as shown by the dotted lines I3.

The grounded antenna circuit I4 iscoupled, as at M1, to the tuned input circuit of Ytube I0. The anode circuit of tube I0 is coupled, as at M2, to the tunable input circuit of the screen grid tube II. The anode circuit of tube II is coupled, as at M3, to the tuned input circuit of diode I which is connected between the junction of the strapped grid and anode on one hand and the cathode on the other hand. It is to be clearly understood, however, that the tube I may be of the type wherein only a cathode and anode `are disposed in the envelope.

The rst stage of audio frequency amplification includes the tube I5, the grid of this tube being resistively coupled by a lead I6 to the high potential terminal of resistor R1, the said high potential terminal being connected to the anode of diode I. The low potential side of resistor R1 is connected by a lead I1 to a desired point on the resistor R3, it being preferable to terminate the lead I1 with'an adjustable sliderso that the bias upon the diode may be readily adjusted.

The resistor R3 has one side thereof grounded, and its other side terminating in lan adjustable Contact I8. A lead I9 is provided for connecting a point on resistor R3, which is of a more positive potential than the setting of slider S, to the cathodes of tubes I0 and II. 'Ihe anode of tube I5 isv arranged for connectionto a succeeding stage of audio frequency ampliiicatiombut a path including condenser 20, and resistor R1 is' connected between the junction of the 'anode of tube I5 and one side of the condenser 20 and the' positive terminal of the plate potential source to provide a positive potential path for the plate of tube I5, as well as to'provide the desired positive direction biasing potential for. the anode of diode I.

The Contact I'8 may be connected toy terminal 2 I, or to terminal 22,"it being rnoted that theterminal 22 is connected by a lead 23 to the lower side of the resistor R4. The lead' 24` is connected to the grid of tube I0 through. a. iilter resistor'26, and tothe grid of tubek II through .a filter resistor 26', the radi'o'frequency by-pass condensers '21 being employed to'by-pass. radio frequency -acetic? currents away from the grids and cathodes of tubes I0, II and I. These filter resistors and by-pass condensers provide the alternating current load R2 of Fig. 1 (b).

In describing the operation of the receiver shown in Fig. 3, it will be noted that two methods of maintaining the anode of diode I biased in a positive sense are provided. When the contact I8 is connected to terminal 22, there is provided an arrangement in which bias would ordinarily and preferably be obtained for the anode of diode I through the bleeder resistor R4. With thecontact I8 thus connected it will be noted that the anode of diode I is maintained at a less negative direct current potential since the cathode of diode I is grounded.

When the contact I8 is moved into position to connect with terminal 2l, it being clearly understood that this mode of alternate connection is merely by way of illustration, there is provided an automatically adjusted bias for the anode of diode I in order to adjust the bias of the anode of diode I to the incoming carrier strength. With the contact I8 thus connected the direct current developed across resistor Ri biases the following tube I5. The higher the carrier strength the greater will be the negative bias on the grid of tube I5, and, accordingly the direct current plate potential of tube I5 will be higher. By properly choosing the values of R4 and R3 with regard to the mu of the tube I5, the bias in the positive direction on the anode of diode I may be made to substantially follow the carrier strength. It will thus be seen that Fig. 3 shows two methods of positively biasing the anode of the diode I with respect to its cathode. In actual construction the receiver preferably will not include a switch I8, but have I8 permanently connected to 22 or to 2l, as the case may be.

The operation of the circuit can best be understood by appreciating the fact that it is desired to maintain the reproducer volume of the receiver shown in Fig. 3 at a substantially constant value.

This is accomplished by maintaining the carrier input intensity to the diode detector I at a substantially uniform level. Should the carrier level vary at the antenna I4, for one reason or another, then it is desired to automatically regulate the gain of the radio frequency tubes I0 and II in such a manner that the carrier variation is compensated for. For example, if the carrier level delivered to the input of the diode rectifier decreases below the predetermined desired level, then the potential drop across the resistor Ri will decrease, with the result that the bias on the grids of tubes I0 and II will decrease.

Since the cathodes of each of tubes IIJ and II are maintained normally, on the resistor R3, at a more positive potential than the grids of tubes I0 and II, this will result in an increase in amplification in the stages including tubes I0 and I I, and .accordingly the carrier intensity level at the input of diode I may be increased to the desired diode input level. On the other hand, if the carrier input to the diode detector should increase above the desired diode input level, the potential drop across the resistor R1 will increase, and the grids of tubes I0 and II will be negatively biased.

It will now be seen that by externally applying a bias in a less negative direction to the diode anode, the distortion shown in Fig. 2 is substan- .tially reduced, and this is so even though the alternating current. load (the automatic gain control circuit) tends to produce the aforesaid distortion. Of course, the present invention is not limited to an alternating current load comprising the automatic gain control system, but may be any other type of circuit which affects the diode detector operation as shown in Fig. 2. Also, the diode'l need not be the detector of a receiver, but may be any rectifier independent of the detector used for gain control.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be employed, without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination with a high frequency amplifier of a radio receiver, a diode rectifier, a load circuit including a potential source coupled to the output of the rectifier, the input of said rectifier being coupled to the output of said amplifier, a biasing path between the anode of said diode rectifier and an input electrode of said amplifier, the amplifier cathode being at a more positive potential than the diode cathode, and additional means for connecting said diode anode to a more positive potential point of the load circuit than the diode cathode, said connecting means including a resistor.

2. In combination with a high frequency amplifier of .a radio receiver, a diode rectifier, a load circuit including a potential source coupled to the output ofthe rectifier, the input of said rectiiier being coupled to the output of said amplifier, a biasing path between the anode of said diode rectifier and an input electrode of said ampliiier, and additional means for connecting said diode anode to a more positive potential point of the load circuit than the cathode, the amplifier cathode being at a more positive potential than the diode anode and cathode, said load circuit including a low frequency amplifier having its input circuit connected to the anode circuit of said diode.

3. In combination with a high frequency amplifier of a radio receiver, a diode rectifier, a load circuit including a source of positive potential coupled to the output of the rectifier, the input of said rectifier being coupled to the output of said amplifier, a biasing path between the anode of said diode rectifier and an input electrode of said amplifier, and a path for connecting said diode .anode to a more positive potential point on the load circuit than the cathode, said second path including a resistor, and said load circuit including a low frequency amplier having its input circuit connected across said resistor.

4. In combination with a high frequency amplifier of .a radio receiver, a diode rectifier, a load circuit including a source of positive potential coupled to the output of the rectier, the input ofy said rectifier being coupled to the output of said amplifier, a biasing path between the anode of said diode rectifier and an input electrode of said amplier, and a path for connecting said diode anode to a more positive potential point on the load circuit than the cathode, the amplifier cathode being at a more positive potential than the diode .anode and cathode, said second path including a resistor.

5. In combination with a high frequency amplifier of a radio receiver, a diode rectifier, a load circuit including a source of positive potential coupled to the output of the rectier, the input of said rectifier being coupled to the output of said amplifier, a biasing path between the anode of said diode rectifier and an input electrode of said amplifier, and a path for connecting said diode anode to a more positive potential point on the load circuit than the cathode, the amplifier cathode being at a more positive potential than the diode anode and cathode, said load circuit including a low frequency amplifier having its input electrodes connected across a portion of said biasing path, said second path including a connection between the diode anode and a predetermined point in the anode circuit of said low frequency amplifier.

6. The method of automatically controlling the gain of a radio frequency amplifier in a radio receiver which consists in impressing modulated carrier energy upon the amplifier,rectifying the amplified energy in a diode stage, causing the direct current component of the rectified energy to develop a biasing potential,` automatically controlling the gain of said amplifier with said biasing potential when the collected `carrier intensity varies from a predetermined level, and applying a positive direct current potential on the anode of the diode rectifier of such a magnitude that distortion due to diode cut-off at high carrier modulations is substantially prevented.

7. The method of diminishing distortion in a diode detection circuit having an alternating current output load tending to produce said distortion consisting in applying signal energy to the diode electrodes of a magnitude such that distortion due to diode cut-off at high carrier modulation occurs, and applying a direct current potential to the anode in a direction such that it will be less negative with respect to the cathode than without said additional potential and of a magnitude such that said distortion is effectively decreased.

8. In a signalling system, an electron discharge tube amplifier having a cathode and a control electrode, an electron discharge tube detector coupled to said amplifier, said detector being of the diode type and having a cathode and a cold electrode, means for maintaining said cold electrode normally negative relative to said amplifier cathode and positive relative to the detector cathode, means including a resistor connected between the diode cathode and cold electrode for causing said detector cold electrode to become more negative in the presence of an amplified signal, and a direct current connection between said control electrode and said detector cold electrode whereby the amplification of said amplifier is automatically regulated.

9. In a signalling system, an amplifier tube having a cathode and a control electrode, a rectifier coupled to said amplifier, said rectifier including an output electrode, means for maintaining said output electrode normally negative relative to said amplifier cathode, means for causing said output electrode to become more negative in the presence of an amplified signal, a direct current connection between said control electrode and said output electrode whereby the amplification of said amplifier is automatically regulated, and additional means for causing said output electrode to become more positive with respect to the rectifier cathode with increase of signal intensity.

`10. In a signalling system, a vacuum tube amplifier having a cathode and a control electrode, a diode rectifier coupled to said amplifier, said rectifier having at least a cathode and an anode, means for maintaining said anode normally positive with respect to the rectifier cathode, said amplifier cathode being maintained at a positive potential with respect to the diode cathode, means for causing said rectifier anode potential to move in a negative direction in the presence of an amplified signal, and a direct current connection between said control electrode and said rectifier anode whereby the amplification of said amplifier is automatically regulated.

11. In combination with a rectified signal repeater diode'rectier having a tuned signal input circuit connected between its anode and cathode, a tubehaving a source of direct current voltage in its anode circuit, an impedance electrically associated with said source, a direct current connection including a resistor between the diode anode and a point on said impedance which is at a positive potential with respect to the diode and` tube cathodes, and means, including a direct current connection between the diode anode side of said resistor and the input grid of the repeater tube, responsive to signals for varying said tube conductivity.

CHARLES TRAVIS. 

